Older people are reported to be chronically dehydrated, although the cause of the inability to maintain euhydration is not well understood. Dehydration results in circulatory and thermoregulatory impairments, particularly during physical activity or heat exposure, and places the individual at an increased risk of cardiovascular and heat-related injury. The overall hypotheses of this project are: A. The relative dehydration in older people is in part the consequence of physical inactivity and is due to deadaptation of the systems involved in maintenance of extracellular fluid volume. B.A physical activity program will result in improvements in body fluid regulation, and thus restore circulatory and thermoregulatory potential. The first aim of the project is to determine the reason(s) for the chronic dehydration in older people and to determine how these are reversed by increased physical activity. We will dehydrate and rehydrate older people to determine the site of deficit in the regulatory circuit. We will repeat the studies during the course of an endurance training program. The second aim is to test the hypotheses that a) a low blood volume restricts the ability of older people to thermoregulate during physical activity or heat exposure and b) increased physical activity induces blood volume expansion, thus improving the ability to thermoregulate during a standard exercise/heat protocol. It is not known whether older people retain the ability to expand blood volume. The third aim is to characterize cardiopulmonary (CP) baroreflex sensitivity in older people before and during a training program. CP baroreflex sensitivity affects arterial blood pressure regulation; therefore, changes in CP baroreflex sensitivity may be involved in the progressive impairment with age. CP baroreflex sensitivity also affects body fluid balance; a high sensitivity, associated with unfit individuals, will result in a lower blood volume. We will test the hypothesis that older people have a high CP baroreflex sensitivity, as do unfit younger people, and that CP baroreflex sensitivity will decrease with increasing fitness. By determining the mechanisms associated with the chronic dehydration in inactive older people and the extent to which these can be changed with activity; and by determining whether the capability for blood volume expansion, and its associated benefits, is retained with advancing age; and by determining whether CP baroreflexes are altered with age or with activity, we hope to provide insight into the mechanisms associated with reductions in function with aging and how improvements in function are induced by increased activity.